Depending on speedy advancement of computer technology, computerized graphics (or digital printing) is the mainstream of modern printing technology. In comparison with complicated processes of conventional printing, digital printing is the reproduction of digital images on a surface of a print substrate (ex. paper) by in-position printing the selected ink pigments injected from lasers or specific nozzles that are controlled by delicate electrical injecting system. Hence, the printout of digital printing (ex. photo) is obtained and has printing quality and dots per inch (dpi) equally as conventional printing quality.
As such for ink jet printing technology, in addition to ink jet printing technology, the digital printing quality of ink jet printing is also improved by the composite printing material. It is important to produce the composite printing material that treats pigments and carriers appropriately during quickly injecting inks. That is to say, the composite printing material can separate pigments and carriers quickly. Moreover, the composite printing material can keep the pigments fixed at the selected location. Furthermore, the composite printing material can match the requirements of quick printing, water resistance, and necessarily glossy or mat gloss. In general, such composite printing material is composed of a printing substrate and a coating layer disposed thereon, for example, laminated paper. It is closely related among the cost, improved printing quality and imaging performance of the composite printing material, and the printing quality and imaging performance mainly depend on the coating layer. Typically, the coating layer includes powder and adhesive in a certain thickness, in which the powder serves to fill up uneven surface of the original printing substrate for smoothing. Various thickness of the coating layer can possess the surface of the composite printing material with glossy or mat gloss. Besides, the coating layer can provide opacity and improved whiteness, and it servers as mainly water and ink absorption capacity.
The powder commonly utilized in the composite printing material may be silica (silicon dioxide), clay, kaolin, calcium carbonate and so on. The powder preferably has the properties of uniformly particle size distribution, high whiteness, high gloss, low cost and good chemical stability as far as possible. The adhesive serves to improve the physical strength, surface performance, and to provide close combination with the coating layer and the printing substrate for preventing powder from coming off, thereby densifying the coating layer. Common adhesive may be poly(vinyl alcohol) (PVA), casein, rosin glue and paraffin glue. The adding amount of the additive depends on the particle size, shape and so on. However, the coating slurry added with too more adhesive is too viscous to spread, and the resulted coating layer has poor ink absorptivity. On the contrary, the coating slurry added with too less adhesive is too thin to fix the coating layer and the printing substrate, and the resulted coating layer suffers powder coming off and even peeling off.
Since the powder of the coating layer is silica (silicon dioxide)-based, and foreign countries control most technologies of powder production. Domestic industries acquire the material of the aforementioned coating layer only through import, and the cost is more expensive. Even though other powder substitutes silica, the resulted composite material hardly matches the requirements of quick printing, water resistance, and glossy surface. In the example of the most available silica-based coating layer applied on the paper, this coating layer hardly matches the requirement of glossy surface. Moreover, the silica is applied in the coating layer, the printout easily diffuses while suffering water, and the silica-based coating layer has poor water resistance. Furthermore, the silica-based coating slurry has less solid content, for example, 15% to 20%, and it consumes more time for coating and drying.
Hence, it is necessary to provide a surface coating slurry of digital printing having low cost, high ink absorptivity, water resistance, glossy or mat surface, so as to overcome the disadvantages of conventionally silica-based coating layer, for example, high technical cost, conventionally complicated cold coating process for forming a water resistance layer, long coating process time, poor printing quality and so on.